“And they contract those muscles at very high rates, and that vibrates the swim bladder, and that in turn generates a vibration in the surrounding water that makes the sound.”

They can make that sound, non-stop, for hours at a time. But once a female arrives, mating begins, and she leaves her eggs, the male stops singing for the night. It is the male that then watches over the eggs until they hatch — and the young fish can swim on their own.

Plainfin midshipman fish in an undated photo.

The fish experiments were done in a Cornell laboratory by then-student Ni Feng, who is now at Yale University. It was her idea to study how melatonin affects the fish. The researchers had no idea how the hormone might affect the fishes’ ability to sing and hum.

All about the rhythm

The fish were kept in rooms where lighting could be controlled. When it was always dark, the fish hummed as they would at night. They followed their internal timing or circadian rhythm.

But in bright lighting, a condition that lowers melatonin production, humming was suppressed. When kept in normal light, but given a melatonin replacement, they continued to hum but at unpredictable times.

So unlike humans, and even birds, the melatonin did not put the fish to sleep. It had just the opposite effect.

“So here we study melatonin in our animal, right? And instead of suppressing calling, or putting them to sleep, essentially, it somewhat acts as a stimulant. Again, it’s the “go” signal that says ok, make more hums...”

He says that by studying how different animals react to melatonin, researchers will get a better understanding of how it works.

“We will reveal very fundamental mechanisms of just how this hormone in fact controls daily sleep/wake cycles in all animals.”

The hum of space aliens?

The midshipman fish are not the only ones making sounds. In fact, long ago, fishermen would listen carefully to hear where the fish were. Bass says he even found a report written in 1924 about the plainfin midshipman and how they hum at night.

In the 1980s, people living on houseboats along the California coast did not know what to make of those sounds. Bass said they thought the sound came from either an underwater power line, a nearby water treatment center, a U.S. Navy experiment, or maybe even beings from other planets.

The boat owners found out the sound was coming from the male midshipman, looking for love in the night.

Like whales and dolphins, other sea creatures, the midshipman fish make different kinds of sounds for social communication. For example, the sound they make when protecting their nests from possible invaders is different than the one they make when calling a mate.

Bass says that in the end, humans and other animals are similar in certain behaviors -- in this case, the ability to produce sounds. He says these fish, and the things he learns about them, never cease to amaze him.

I’m Anne Ball.

Anne Ball reported this story for Learning English with information from Reuters News Service. George Grow was the editor.